1. Field of the Invention
The present invention relates to an apparatus in which two-dimensionally scanning is effected on a recording medium using a probe located close to the recording medium to perform recording/reproduction of information by utilizing a physical interaction induced between the recording medium and the probe and, more particularly, to a small, high-density information recording/reproducing apparatus taking advantage of the principle of an STM.
2. Related Background Art
Recently, a scanning tunnel microscope (to be referred to as an STM hereinafter) capable of directly observing the electronic structure of a surface atom of a conductor has been developed (G. Binning et al., Phys. Rev. Lett. 49 (1982) 57), and this makes it possible to measure a real gap image at an extremely high resolution on the order of nm or less regardless of whether the image is single-crystal or amorphous. This STM observes the surface condition of a conductive sample by using a tunnel current that flows when a metal probe and the sample are moved close to each other to a distance of about 1 nm with a voltage applied between them. This current is very sensitive to the change in distance between the probe and the sample. Therefore, the surface condition of the sample can be observed with a resolution on the order of an atom by scanning the probe on the sample with the tunnel current maintained constant and measuring the change in distance between the probe and the sample, or by measuring the change in tunnel current when the scan is performed with the distance maintained constant.
Although the analysis using the STM is conventionally limited to conductive materials, the STM has begun to be applied to an analysis of the structure of a thin insulating film formed on the surface of a conductive material. In addition, as the STM techniques have been developed, a technique has been proposed which measures the surface condition of a sample by scanning a probe on the sample while detecting various physical interactions, not limited to a tunnel current, produced between the probe and the sample that are located very close to each other. An apparatus or means of this type not only has a high spatial resolution but makes use of a method of detecting a micro signal, such as a tunnel current. This brings about an advantage of the ability to observe a sample with a low power without damaging it. Furthermore, since the apparatus can operate in the open atmosphere, extensive applications are expected.
In particular, as disclosed in Japanese Laid-Open Patent Application Nos. 63-161552 and 63-161553, many efforts have been made to use these techniques in high-density recording/reproducing apparatuses. That is, a probe is used to physically deform a recording medium corresponding to a sample or to change the electronic condition on the surface of the recording medium, thereby recording information, and information in a recording bit is reproduced by an electric current flowing between the probe and the recording medium. The use of this method makes it possible to perform recording/reproduction of a large quantity of information at a high density, on the order of a molecule or an atom.
In the above recording method, in order to cause a physical change, a recording probe with a sharp tip is urged against a recording medium to recess it. It is also reported recently that, on a recording medium consisting of, e.g., graphite, very small holes about 4 nm in diameter can be formed by application of a pulse voltage with a peak value of 3 to 8 V and a pulse width of 1 to 100 .mu.s. These holes are sufficiently usable as recording bits. As a method to perform recording by changing an electronic condition, on the other hand, there is a known method in which a voltage is applied across a recording medium and a probe electrode to change the electrical characteristics of a micro region. This method has attracted attention because it facilitates erasure and rewriting of information. A structure usable as a recording medium is, for example, a thin-film layer of a material that exhibits switching characteristics with a memory effect in a voltage-current characteristic, such as a chalcogenide or a .pi. electron-based organic compound. An example is a structure in which a multi-layered film of an appropriate organic substance is formed on a base electrode by using a Langmuir-Blodgett method (to be referred to as an LB method hereinafter).
A probe unit is commonly a structure in which a metal needle consisting of, e.g., Pt, Pt-Ir, or W, whose tip is mechanically polished and then electropolished, is attached to a piezoelectric element. In this structure, a displacement is controlled by a voltage applied to the piezoelectric element. Techniques of semiconductor manufacturing processes can also be used. That is, formation of a probe unit having a fine structure is also possible by the use of a processing technique (e.g., K. E. Peterson, "Silicon as a Mechanical Material," Proceedings of the IEEE, Vol. 70, page 420, 1982) that can form fine structures on a single substrate.
In order for an information recording/reproducing apparatus of the above sort to function correctly, it is essential that a probe be located close enough to the surface of a recording medium to produce a physical interaction, such as a flow of a tunnel current, between them. For this purpose, a probe approaching mechanism must be used to approximate the distance between the probe and the recording medium to the distance described above. Conventionally, a stepping motor, an inch worm, or a stacked piezoelectric element is used as this approaching mechanism. The approaching mechanism is controlled by monitoring a signal, such as a tunnel current, detected as a position signal by the probe, thereby approaching the probe to a desired position, as described in (Appl. Phys. Lett., 40 (1982) 178).
The above conventional example, however, has the following problem; a signal from the probe can be detected only within a very limited region, such as a distance of nm or less, in which a physical interaction occurs between the probe and the recording medium. If the prove moves too close, even slightly, to the recording medium, a possibility of collision arises. A collision of the probe not only damages the recording medium but decreases the spatial resolution due to an increase in radius of curvature at the tip of the probe. In the above method and means, therefore, the gain of an approach control system must be set low so that neither overshoot nor ringing occurs in the system. Consequently, it becomes difficult to improve the response speed of control, i.e., to complete the approach of the probe in a short time period.